In the mid-1980s, a diver was exploring the Ryukyu Islands, off the southern coast of Japan. What he didn't expect was to stumble across one of the archaeological mysteries that still intrigue geologists today. Yonaguni is surrounded by theories about its origin: was it natural or was it built by humans? Its rectilinear and symmetrical shapes are what led some to believe the monument could be a megalith built at the end of the last ice age.